In early 2013, Saudi Arabia began smuggling weapons it had purchased from the Croatian government through Jordan to the south of Syria, to forces loyal to the Free Syrian Army. At the time, videos posted by the Syrian opposition showed a number of weapons coming into the south of Syria, including RBG-6 grenade launchers, M60 recoilless guns, RPG-22 rocket launchers, and M79 Osa rocket launchers. In March 2013, videos began to appear showing these weapons appearing with groups outside of the FSA, including Jihadists such as Ahrar al-Sham, and images posted today show these weapons have now spread even further afield.
A series of images were posted on Twitter by the account Alanbar_news, an account used by the al-Qaeda linked Islamic State of Iraq and al-Sham, with the @Alanbar_news account covering their operations in Al Anbar Governorate of Iraq. The images show their operations against US-made armoured vehicles belonging to the Iraqi army (collected here), and one image in particular stood out
This photograph appears to show a M79 Osa, as supplied to the Free Syrian Army at the start of 2013, being used to attack Iraqi armoured vehicles inside Iraq. The following image compares the M79 Osa to what's visible in the above photograph
Another recent photograph from ISIS shows a fighter with what appears to be a Croatian RBG-6 grenade launcher
These weapons are not used in the region (apart from those supplied to the Free Syrian Army), so it seems extremely likely these weapons are the same ones purchased from Croatia, by Saudi Arabia, and smuggled through Jordan. Now, despite the best intentions of those who were involved with providing these weapons to the Free Syrian Army, these weapons are being used to attack Iraqi government forces inside Iraq, a clear indication that attempts to control the spread of weapons beyond the borders of Syria and the groups that initially received them has failed in a spectacular fashion. This raises serious questions when there's talk of providing the opposition in Syria with Chinese FN-6 MANPADS (surface-to-air missiles), especially when these Croatian weapons have ended up in the hands of the Islamic State of Iraq and al-Sham.
Thanks to @ShamiWitness for highlighting these photographs, and Aymenn Jawad al-Tamimi for additional information.